1. Field of the Invention
The invention concerns an exhaust gas turbocharger for an internal combustion engine.
2. Related Art of the Invention
From the publication DE 196 15 237 C2 an exhaust gas turbocharger of this general type is known, having a turbine with a radial and a semi-axial flow intake cross-section in the exhaust flow area of the turbine. The flow intake profiles, between which a flow promoting contoured flow ring is provided in the flow intake area of the turbine, makes possible both a radial and also a semi-axial impinging onto the turbine wheel. In the radial flow entry cross-section a variable geometry arrangement is provided with adjustable guide vanes, via which the flow entry cross-section can be varied. By adjustment of the guide vanes the gas pressure, as well as the type and manner of the flow of the exhaust gas onto the turbine wheel, can be influenced, whereby the performance of the turbine and the output of the compressor can be adjusted depending upon the requirements and operating condition of the internal combustion engine.
This type of exhaust gas turbocharger, having variable turbine geometry, is employed also in braking operation of the internal combustion engine. In the braking operation the guide vanes are adjusted into a blocking or choking position, in which the intake cross-section is significantly reduced, whereupon an elevated exhaust pressure builds up in the conduit upstream of the turbine, which brings about, that the exhaust gas flows with increased velocity through the channels between the guide vanes, whereupon the turbine wheel is impinged with a stronger impulse. This brings about an elevated compressor output, so that the fresh or combustible air reaching the motor is also placed under an elevated charge pressure. The cylinder is acted on with increased charge pressure on the inlet side, at the same time the exhaust side is experiencing elevated exhaust gas pressure, which opposes the evacuation or exhausting of the compressed air via the brake value in the exhaust gas conduit. During motor operation the piston in the compression and exhaust stroke must perform compression work against the high overpressure in the exhaust side, whereby a strong brake effect is achieved.
The desired high brake power can however only be achieved when a desired pressure distribution exists within the turbine and when the exhaust gas flows through the turbine in the intended manner. It is a problem herein that leakages occur on the axial sides of the adjustable guide vanes, which can occur due to construction and manufacturing tolerances, however also due to wear and thermal expansion, and can strongly compromise the desired pressure relationship within the turbine, which negatively influences the motor brake power, and however also negatively influences the motor power in the combustion drive mode. This type of guide vane leakage results also from gaps inherently required in construction to enable movement of the guide vanes of the guide vane ring of the variable turbine geometry in the flow entry cross-section.
Similarly, from the publication DE 39 41 399 C1 an exhaust gas turbocharger for an internal combustion engine is known, which is equipped with a twin flow spiral channel with radial and semi-axial flow entry cross-section in the turbine housing, wherein the two flow channels are separated by a fixed separating wall. Between the radial and the semi-axial flow entry cross-section of the two flow channels there is, in the area of the end surface of the separating wall separating the two flow channels, an axially adjustable slider, which is adjustable between a position blocking the radial inflow cross-section and a position blocking the semi-axial inflow cross-section. The slider assumes the function of a variable geometry turbine part, via which the flow behavior of the flow onto the turbine wheel is to be influenced. Even with this turbocharger design, flow leakage or by-pass cannot be prevented.
The publication DE 35 41 508 C1 discloses an exhaust gas turbocharger with radial flow entry cross-section towards the turbine wheel, wherein in the flow entry cross-section a guide ring with adjustable guide vanes is provided. Two holder- or mount-rings engaging the guide vanes on their end surfaces are connected to each other via multiple screws distributed about the circumference. The screws are within spacer sleeves, which ensure a minimal separation of the two mounting rings. An axial relative movement of the outer support rings relative to the inner support ring is not possible on the basis of the screw connection, and namely neither in the direction of a larger separation of the support rings nor in the direction of a coming together of the support rings. This has only the consequence, that the gap between the axial end surfaces of the vanes of the guide vane assembly and the two support rings are arranged with fixed, non-changeable dimensions. Therein a compromise is entered into between having a sufficiently large degree of movement for the blades and a sufficiently small gap for avoidance of by-pass flows. Thermal expansion in the construction components can lead within the turbocharger to an enlargement of the gaps and thereby bring about undesired increase in leakage with correspondingly smaller compressor output.
The publication DE 100 29 640 A1 discloses an exhaust gas turbocharger with semi-axial and with radial flow entry cross-section to the turbine wheel which are separated by an axially displaceable flow ring. In the radial flow entry cross-section a guide vane ring with adjustable guide vanes and in the semi-axial cross-section a grid with fixed geometry are provided. If the guide vane ring in the radial cross-section is moved into the choke or blocking position, then a larger proportion of the exhaust gas flows through the semi-axial cross-section. Aerodynamic effects can be caused by the displacement of the flow ring in the direction of the radial ring of guide vanes.